Eight Libyan pro-government fighters and at least 10 Islamic State (IS) group militants were killed in clashes on Sunday as some militants tried to flee their besieged stronghold, the unity government said.

The fighting since the offensive began on 12 May has left more than 450 pro-government fighters dead and 2,500 wounded. The IS death toll is not known. An IS defeat in Sirte would be a serious blow to the group, which has faced major setbacks in Iraq and Syria in recent months.

Pro-government forces said they were "progressing" in Sirte, where they have surrounded IS militants, prompting some to try to escape.

They said pro-government fighters had recovered the bodies of at least 10 militants and were chasing another group that had fled the coastal town, 450km east of the capital Tripoli.

A Dutch photojournalist covering the government-backed offensive against IS also died, a doctor said.

Dr Akram Gliwan, spokesman for a hospital in Misrata, where pro-government fighters are treated, told AFP that photographer Jeroen Oerlemans was "shot in the chest by an IS sniper while covering battles in Sirte.”

Gliwan said his body had been transferred to Misrata, 200km west of Sirte.

Oerlemans was working in Libya for a number of organisations, including the Belgian weekly Knack magazine, which confirmed his death.

A message on Knack's website said Oerlemans was shot on a reporting assignment and that the publication "wishes his family much strength".

Eight soldiers loyal to the Government of National Accord (GNA) were killed, according to the Facebook page of a field hospital set up by their forces in Sirte.

Twenty-four members of the pro-GNA forces were wounded and transferred to the main hospital in Misrata.

Forces allied with the GNA have cornered the group in Sirte, the birthplace of ousted dictator Muammar Gaddafi.

But suicide bombings and sniper fire from the surrounded militants have slowed the assault.

On Saturday, GNA aircraft conducted six sorties in preparation for an advance on the militants' hideout in the city's east, said the GNA statement.

American aircraft have also carried out 177 air strikes since early August in support of GNA forces, according to US Africa Command.